Is the global primary production at it’s maximum rate?

25 February 2020

Lauderdale and co-authors (2020, see reference below) are seriously questioning the paradigm establishing that marine phytoplankton growth is limited by iron on a global scale.

Iron availability to marine microbes is facilitated by binding with organic molecules which, in turn, are produced by microbes. The authors hypothesize this forms a reinforcing cycle between biological activity and iron cycling that locally matches the availability of iron and other nutrients, leading to global-scale resource co-limitation between macronutrients and micronutrients, and maximizing biological productivity – adding additional iron to the ocean, even in iron-limited regions does not increase overall productivity. Idealized models support this hypothesis, depending on the specific relationships between microbial sources and sinks of organic chelating molecules. The main outcomes of this work are that:

the marine ecosystem appears to have optimized ligand properties through evolutionary selection, beginning at the Neoproterozoic great oxidation event, when the ocean transitioned from an anoxic conditions with abundant free iron to an oxygenated ocean with only trace concentrations of organic-bound iron.
It also confirms that geoengineering by iron fertilization appears to be ineffective in taking up additional carbon, because global primary production is already at it’s maximum rate.

**Figure:** The «Ligand-iron-microbe» feedback maintains «just enough» iron in the ocean through chelating ligand production by biological activity that depends on iron abundance (top). This feedback is arrested when other resources required for growth, such as macronutrients or light, become limiting. Varying the ratio of ligand production to ligand loss rates over a wide range can generate three nutrient regimes (bottom): the “modern” regime with intermediate ligand lifetime and global-scale colimitation between iron in HNLC regions and macronutrients in oligotrophic gyres, a long-lived ligand regime with an iron-replete global ocean and near complete macronutrient drawdown, and an iron-limited short-lived ligand regime where scarce chelators cannot maintain a standing stock of iron and macronutrients remain unused. Global colimitation (bottom left), with regional limiting resource variation, is subject to a reinforcing feedback if ligand characteristics are in the appropriate regime, but it is not inevitable. Integrated export production (bottom right) reaches its maximum rate within the globally colimited regime, with unused macronutrients from the iron-limited region fuelling phytoplankton growth in the iron-replete region, and chelated iron being returned to the iron-limited regime via the Deep Ocean, fuelling partial macronutrient drawdown there. Adding additional iron to the “Southern Ocean” does not increase net production because the overall availability of resources has been tuned by marine microbes. The vertical dashed line indicates the nutrient regime of the real ocean, found by quantitative comparison of simulated nitrate, iron, and ligand concentrations to available observations in the GEOTRACES IDP v2017. Click on the figure to view it larger.

Reference:

Lauderdale, J. M., Braakman, R., Forget, G., Dutkiewicz, S., & Follows, M. J. (2020). Microbial feedbacks optimize ocean iron availability. Proceedings of the National Academy of Sciences of the United States of America. DOI: https://doi.org/10.1073/pnas.1917277117

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